1. Field of the Invention
The present invention relates to a superheterodyne receiver.
2. Description of the Prior Art
Some receivers based on double superheterodyne reception are known to vary both first and second locally generated frequencies for tuning in to stations.
One of these double superheterodyne receivers will be described for example by referring to FIG. 1. In this example, the following frequencies have the following corresponding values:
Received frequency f11=150 kHz to 30 MHz (approximately) PA1 First intermediate frequency f13=55.845 MHz PA1 Second intermediate frequency f15=455 kHz
A first locally generated frequency is varied in steps of 1 kHz, while a second locally generated frequency is varied in steps of 100 Hz to vary the received frequency f11 in steps of 100 Hz.
More particularly, a received signal S11 is supplied from an antenna 11 to a first mixer 13 through a high-frequency amplifier 12. At the same time, a first locally generated signal S31 is supplied from a first local oscillator 31 to the mixer 13. A frequency f31 of the locally generated signal S31 relative to the received signal f11 is as follows: EQU f31=f11+f13 (1)
therefore, EQU f31=(150 kHz+55.845 MHz) to (30 MHz+55.845 MHz) (2)
where the frequency f31 varies in steps of 1 kHz.
Thus, the received signal S11 is frequency-converted by the first locally generated signal S31 to a first intermediate-frequency signal S13 (the intermediate frequency f13) in the mixer 13.
The resultant signal S13 is then supplied through a first intermediate-frequency amplifier 14 to a second mixer 15. At the same time, a second locally generated signal or beat-frequency oscillator signal S41 is supplied from a second local oscillator 41 to the second mixer 15. In this case, a frequency f41 of the locally generated signal S41 is as follows relative to the received frequency f11: EQU f41=f13-f15 EQU =f31-f11-f15 (3)
therefore, EQU f41=(55.39 MHz+0.5 kHz) to (55.39 MHz-0.4 kHz) (4)
where the frequency f41 varies in steps of 100 Hz.
Thus, the first intermediate-frequency signal S13 is frequency-converted by the second locally generated signal S41 to a second intermediate-frequency signal S15 (the intermediate frequency f15) in the mixer 15.
The resultant signal S15 is then supplied through a second intermediate-frequency amplifier 16 to an AM detector 17 where an audio signal is demodulated to be sent through a switch circuit 18 to a terminal 19.
When receiving an SSB signal, the intermediate-frequency signal S15 is partially taken from the second intermediate-frequency amplifier 16 to be supplied to a balanced mixer 21. At the same time, a BFO signal S22 having a frequency f22 of 453 kHz for example is taken from a BFO 22 to be supplied to the balanced mixer 21.
Consequently, an audio signal demodulated from the SSB signal is taken from the mixer 21 and, upon reception of the SSB signal, supplied through the switch circuit 18 to the terminal 19.
According to the above constitution, the received frequency f11 is as follows from the equation (3): EQU f11=f31-f41-f15 (5)
Since the frequencies f31 and f41 vary, relative to the received frequency f11, inside the respective ranges given by the equations (2) and (4), the received frequency f11, when f31=150 kHz+55.845 MHz and f41=55.39 MHz+0.5 kHz for example, is f31-f41 -f15=(150 kHz+55.845 MHz)-(55.39 MHz+0.5 kHz)-455 kHz=150 kHz-0.5 kHz.
When f31=30 MHz+55.845 MHz and f41=55.39 MHz-0.4 kHz for example, the received frequency f11=f31-f41-f15=(30 MHz+55.845 MHz)-(55.39 MHz-0.4 kHz)-455 kHz=30 MHz+0.4 kHz. Therefore, the receiving band is approximately 150 kHz to 30 MHz.
According to the equation (5), varying the first locally generated frequency f31 in steps of 1 kHz with the second locally generated frequency f41 being kept constant varies the received frequency f11 in steps of 1 kHz, while varying the second locally generated frequency f41 in steps of 100 Hz with the first locally generated frequency f31 being kept constant varies the received frequency in steps of 100 Hz.
Therefore, respectively varying the first locally generated frequency f31 and the second locally generated frequency f41 varies the received frequency f11 within the receiving band of 150 kHz to 30 MHz in steps of 100 Hz.
As shown in the equation (5), the received frequency f11 of the double superheterodyne receiver of this example is determined by both the first locally generated frequency f31 and the second locally generated frequency f41. Therefore, the first locally generated frequency f31 and the second locally generated frequency f41 must be accurate enough and be able to vary in accordance with the received frequency f11.
To implement this accuracy and ability to vary the frequency, conventional double superheterodyne receivers including the one mentioned above constitute the local oscillators 31 and 41 with a phase-locked loop (PLL) each.
However, since the PLL is expensive, implementing the local oscillators 31 and 41 with PLL's makes receivers expensive.
Even if local oscillators 31 and 41 are constituted with PLL's, the errors and/or fluctuations in the locally generated frequencies f31 and f41 cannot be fully eliminated. And even if they are negligible for the individual frequencies, these errors and/or fluctuations are compounded as shown in the equation (5) to an extent at which the resultant errors and/or fluctuations may not be ignored any more. It is therefore required to arrange the two PLL's so that the errors and/or fluctuations in the locally generated frequencies f31 and f41 if any are canceled in the equation (5), which also results in a costly receiver.
The present invention is intended to solve the above-mentioned problems.
Now, when the locally generated frequency f31 of the local oscillator 31 is made equal to the first intermediate frequency f13, the resultant locally generated signal S31 is supplied through the mixer 13 and the intermediate-frequency amplifier 14 to the mixer 15.
Since the locally generated signal S41 is present at the mixer 15, the locally generated signal S31 supplied to the mixer 15 is frequency-converted by the locally generated signal S41 to the intermediate-frequency signal S15 having the frequency f15 obtained as f15=f31-f41=f13-f41. The resultant signal S15 is supplied through the intermediate-frequency amplifier 16 to the mixer 21.
At the mixer 21, the signal S15 supplied to it is beaten down by the BFO signal S22 (frequency f22), so that a signal S21 having a frequency f21 obtained as f21=f15-f22=f13-f41-f22 is taken from the mixer 21. For example, if the frequency f41 of the locally generated signal f41=(55.39 MHz+0.5 kHz), then f21=55.845 MHz-(55.39 MHz+0.5 kHz)-453 kHz =1.5 kHz; if f41=55.39 MHz, then f21=55.846 MHz -55.39 MHz-453 kHz=2 kHz; and if f41=(55.39 MHz -0.4 kHz), then f21=55.845 MHz-(55.39 MHz-0.4 kHz) -453 kHz=2.4 kHz. That is, if value n is an integer between -5 and +4 inclusive and f41=(55.39 MHz-n.times.0.1 kHz), then f21=55.845 MHz-(55.39 MHz-n.times.0.1 kHz)-453 kHz=(2+n.times. 0.1) kHz.
Consequently, if, at the time of frequency calibration:
1) the local oscillator 31 is constituted by a PLL;
2) the frequency f31 generated by the local oscillator 31 is made equal to the first intermediate frequency f13;
3) the local oscillator 41 is constituted by a variable frequency oscillator;
4) a control voltage Vn of the local oscillator 41 when the frequency f21 of the signal S21 is checked and found to be (2+n.times.0.1) kHz is stored in memory; and
5) the control voltage Vn stored in memory in 4) above is supplied to the local oscillator 41 at the time of reception; then the frequency f41 generated by the local oscillator 41 is calibrated, thereby realizing reception without frequency errors and/or fluctuations.